This invention relates to tensioning devices as utilized in maintaining constant torque on pulley assemblies employed in the movement of tapes or strands. More particularly, the invention relates to tensioning devices which include a permanent magnet and a hysteresis flux ring, wherein as the hysteresis ring rotates, the poles of the magnet generate a "drag" or retarding torque on the pulley assembly.
Of critical importance to the satisfactory operation of such devices is the setting and maintenance of an air gap or spacing between the hysteresis ring and the magnet. Air gap adjustment mechanisms in prior devices have been complex, bulky, and difficult to maintain. Also particularly in aviation and outer space applications, size and weight limitations have generated a specific need for compactness in tension control devices. To date, such requirements have been met through the use of miniature friction clutches, which are unreliable and subject to high failure rates due to wear of parts.
Also of particular concern in aviation and outer space applications are the wide temperature variations encountered. In traditional hysteresis tensioning devices, the hysteresis ring has operated in an unsaturated magnetic state or condition, resulting in performance quite sensitive to temperature. As a result the drag or resistance torque on the pulley assemblies employed in such systems has been variable as a function of temperature to an objectionable degree. Although it has been known that a fully saturated magnetic condition would alleviate the problem, the state of the art has been such that materials for magnets and hysteresis rings have not been suitable for practical use in the latter environments.
Finally, as to use of magnetic tape in tape recording devices, the avoidance of alternating magnetic flux leakage becomes critical whenever magnets are rotated relative to the magnetic tape heads employed in such systems. The movement of the magnets produces extraneous and objectional tape noise due to generation of alternating magnetic flux within the system. Fixation of the magnets relative to the heads would avoid magnetic movement relative to such heads, and thereby result in systems experiencing only non-cyclic or constant magnetic flux leakage, the latter of which is not disruptive.